Anisotropic yielding of injection molded polyethylene: experiments and modeling

The anisotropic yielding of injection molded polyethylene is discussed, specifically focusing on the differences between the tensile and compressive yield stress as a function of loading angle and strain rate. For the first time, it is demonstrated that a strong Bauschinger effect exists in polymers that possess molecular orientation due to melt-processing. A macroscopic constitutive model is proposed to capture the yielding phenomena observed in the experiments. This model features two sources of anisotropy, the physical significance of which is discussed: a frozen-in stress originating from the oriented elastic network, and an intrinsically anisotropic viscoplastic flow rule based on the yield function of Hill, extended to incorporate the asymmetry between the tensile and compressive response. Model simulations demonstrate that the constitutive relation proposed accurately captures the important features of the experimental data. Also, it is able to predict the anisotropy in the failure kinetics of injection molded polyethylene subjected to a constant tensile load in different directions without the requirement of additional parameters.